Absolute lung volume is a key parameter in pulmonary physiology and diagnosis but is not easy to measure in the live individual. It is relatively straightforward to measure the volume of air which is exhaled from a subject's mouth but at the end of complete exhalation, a significant amount of air is always left in the lungs because the mechanical properties of the lungs and chest wall, including the ribs, do not allow the lungs to collapse completely. The gas left in the lungs at the end of a complete exhalation is termed the Residual Volume (RV) which may be significantly increased in disease. The total volume of gas in the lungs at the end of a maximal inspiration is termed the Total Lung Capacity (TLC) which includes the RV plus the maximum amount of gas which can be inhaled or exhaled and which is termed the Vital Capacity (VC). However, during normal breathing the subject does not empty the lungs down to RV nor inflate them to TLC. The amount of gas in the lungs at the end of a normal breath, as distinct from a complete exhalation, is termed the Functional Residual Capacity (FRC) or Thoracic Gas Volume (TGV), depending upon the manner in which it is measured. For simplicity when this volume is measured by inert gas dilution techniques it will be termed FRC and when measured by barometric techniques involving gas compression as in this application it will be termed TGV.
In order to determine the total volumes of gas in the lungs at TLC, TGV or RV, indirect methods must be used since it is impossible to completely exhale all the gas from the lungs. There are two basic techniques currently available, gas dilution and whole body plethysmography (a barometric method). Gas dilution involves the dilution of a known concentration and volume of inert gas by the gas in the lungs of the subjects and is critically dependent on complete mixing of the marker gas and lung gas. In subjects with poor gas mixing due to disease, this technique is very inaccurate and generally underestimates the true FRC. In the whole body plethysmograph, the subject makes respiratory efforts against an obstruction within a gas tight chamber and the changes in pressure on the lung side of the obstruction can be related to the changes in pressure in the chamber through Boyle's law to calculate TGV. This method accurately measures TGV even in sick subjects but requires complicated and expensive equipment and is difficult to perform.
Once FRC (gas dilution), or TGV (whole body plethysmograph), is calculated, the measurement by spirometry of the extra volume of gas which can be exhaled from the end of a normal exhalation (Expiratory Reserve Volume, ERV) and the extra volume which can be inhaled from the end of a normal exhalation (Inspiratory Capacity, IC) allows the calculation of TLC and RV.
These three important indicators (TLC, RV and FRC or TGV) are mutually connected through the following formulas: RV=FRC−ERV and TLC=FRC+IC and, TLC=RV+ERV+IC=RV+VC.
If FRC is determined by gas dilution and TGV by a barometric method, then the difference between them (TGV minus FRC) is a measure, albeit approximate, of the volume of poorly ventilated or ‘trapped gas’ in the lungs.
In healthy subjects TGV and FRC should be virtually identical as there is little or no trapped gas, hence, for all practical matters, the term TGV shall apply for FRC as well. In summary, determination of TLC, TGV and RV is central to the complete evaluation of lung function.
At the present time, FRC is measured by two gas-based techniques: the rebreathing of an inert gas, such as helium, in a closed circuit or the wash in or out of an inert marker gas, which can be the nitrogen, normally present in the lung. Both techniques have been used for several decades and are known to have several shortcomings, e.g., they are complex, hard to operate, moderately expensive, unreliable for the measurement of FRC in patients with poor gas mixing due to disease, and the tests are lengthy and uncomfortable for the subjects.
Body plethysmograph devices for determination of TGV are disclosed, for example, in U.S. Pat. No. 6,113,550 to Wilson, and have been known and used since at least 1955. Other devices, which include the use of impedance belts have been disclosed as well, for example, in U.S. Pat. No. 5,857,459. In both types of devices, the plethysmograph chamber or the impedance belts are designed so that the volume in the lungs can be calculated directly, so as to provide reliable measurement parameters for calculation of TGV. However, these methods for measuring TGV are all less than optimal, requiring a sealed chamber in which the subject sits, or external belts which have been shown not to provide reliable results and which may be bulky, expensive and inconvenient to operate, and require full cooperation of the subject during the measurement maneuvers to obtain meaningful results.
Hand held devices for measurement of certain lung parameters, such as spirometers, are known in the art. However, spirometers are not designed to measure internal volume. Other hand held devices known in the art include devices which have been used to determine airway resistance. Such devices use a shutter mechanism for blocking and opening of airways. For example, U.S. Pat. No. 5,233,998 to Chowienczyk discloses an apparatus with an interrupting valve for interrupting the flow of air through a bore. However, since this device is designed to measure resistance to air flow rather than lung volume, the shutter speeds may be relatively slow, and relative air displacement may occur.
The importance and need for a new, accurate, and easy to use method and device to measure TGV have been clearly stated in the ATS (American Thoracic Society)/NHLBI (U.S. National Heart, Lung and Blood Institute) Consensus Statement of Measurement of Lung Volumes in Humans, Clausen and Wagner et al., Nov. 12, 2003 (the Consensus Statement), page 6: “Systems will be available in the future which through new technology will offer potential advantages (e.g., ease of use, rapidity of testing, improved accuracy) over the methodology recommended in this document (i.e., nitrogen wash-out, helium gas dilution and body plethysmography). The ATS and the ERS (European Respiratory Society) encourage such innovation. However, it is the responsibility of the manufacturers to demonstrate that the lung volumes reported by new technology do not differ substantially from those obtained by the standard techniques; such comparisons must be made using subjects with varying severities of obstructive and restrictive lung disease as well as healthy subjects.”
It is thus an object of the present invention to provide systems and methods for measurement of TGV without the need for external belts or chambers and which can provide accurate measurements which are up to the standards of the currently used systems.